Isomerism in Coordination Compounds
Coordination compounds of the same formula can differ in the spatial arrangement or connectivity of their ligands, giving rise to geometric, optical, linkage, and other isomers with distinct properties.
Definition
Isomerism in coordination compounds is the existence of two or more distinct compounds that share the same chemical formula but differ in the spatial arrangement (stereoisomers) or in the connectivity (structural isomers) of the ligands around the metal centre.
Scope
This topic covers the types of isomerism shown by coordination compounds: stereoisomerism, including cis/trans and fac/mer geometric isomers and the chiral optical isomers of octahedral and other complexes; and structural isomerism, including linkage, coordination, ionization, and hydrate isomers. It treats how isomers are distinguished and why their existence supported the coordination theory, but leaves bonding models and reaction mechanisms to other topics.
Core questions
- What geometric isomers are possible for a given coordination geometry?
- When is a metal complex chiral, and how is optical activity demonstrated?
- How do linkage, ionization, and coordination isomers differ in connectivity?
- How did the existence of isomers support Werner's coordination theory?
Key concepts
- cis and trans isomers
- fac and mer isomers
- Optical isomers and chirality
- Linkage isomerism
- Ionization and hydrate isomerism
- Coordination isomerism
Key theories
- Geometric isomerism
- Ligands at fixed coordination positions can occupy adjacent or opposite sites, giving cis/trans isomers in square-planar and octahedral complexes and fac/mer isomers in octahedral MA3B3 species with different properties.
- Optical isomerism and chirality
- Octahedral complexes such as tris-chelates lack an improper symmetry axis and exist as non-superimposable mirror images; Werner's resolution of such complexes proved that metal centres can be genuinely chiral.
- Structural isomerism
- Compounds of identical formula can differ in connectivity through linkage, ionization, hydrate, and coordination isomerism, reflecting which atom of an ambidentate ligand binds or how ions distribute between coordination sphere and lattice.
Clinical relevance
Isomerism matters in practice because geometric and optical isomers of metal complexes can have different reactivity and biological activity, as in the contrast between the active cis and inactive trans isomers of the platinum drug used in cancer therapy.
History
The number and type of isomers a complex displayed was central evidence in the debate between Werner's coordination theory and Jørgensen's chain theory. Werner's 1911 resolution of an optically active cobalt complex, and later of one containing no carbon, decisively confirmed that complexes have definite three-dimensional structures.
Key figures
- Alfred Werner
- Sophus Mads Jørgensen
- Edith Humphrey
Related topics
Seminal works
- werner1911
- weller2018
- cotton1999
Frequently asked questions
- Why can octahedral complexes be optically active when simple inorganic salts are not?
- When chelating ligands wrap around an octahedral metal they can produce an arrangement lacking any mirror plane or improper axis, so the complex and its mirror image are non-superimposable, exactly the condition for optical activity.
- What is a linkage isomer?
- A linkage isomer arises when an ambidentate ligand, such as nitrite, can bind through either of two different donor atoms—through nitrogen or through oxygen—giving two compounds of the same formula but different metal–ligand connectivity and properties.